CBM GLOSSARY

Adsorption.—The process whereby gas adheres to the internal surfaces of a coal. At low to moderate pressures—typically found at normal exploration depths of less than 2 km—this layer of gas is one molecule thick. At low pressure, there is little difference between total and adsorbed gas.

Adsorption isotherm.—The gas retention capacity of a powdered coal sample at a constant temperature but at different pressures. Usually measured under equilibrium moisture conditions.

Alluvial plain.—A level or gently sloping land surface that is covered by extensive deposition of alluvium.

Ash yield.—Non-combustible inorganic residue remaining after a coal has been completely combusted. It represents the bulk of mineral matter in a coal after carbonates, sulfides, and clays are broken down during heating. Therefore, ash yield is less than the total mineral-matter content.

Biogenic gas.—Gases produced from coal by the metabolic activities of microbes.

Canister.—A gas-tight container for holding a sample as it is being desorbed.

Carbonaceous mudston (carbonaceous shale).—A dark-gray or black rock that contains an abundant amount of carbon in the form of small particles of organic mater; it is commonly associated with coal seams.

Channel sample.—A channel of consistent volume is cut across a seam and all coal within the cut is collected for analysis.

Clastic wedge.—A concave asymmetrical layer of sediments that forms from a domination of sediment input form one side of the basin, generally due to a topographic high or increased sediment input.

Cleats.—Orthogonal sets of fractures in coal caused by shrinkage related to desiccation, devolatization, and structural processes. Somewhat analogous to joints in other rocks.

Coal.—A rock dominantly composed of sedimentary organic matter. A coal contains more than 50 percent by weight and more than 70 percent by volume of sedimentary organic matter.

Coal-bed gas.—Gas produced from the desorption of coal. It is usually composed of methane, but carbon dioxide, nitrogen, and light alkane hydrocarbons are also commonly found.

Coal-bed methane (CBM).—Strictly referring to the methane produced from coalbed gas.

Coal matrix.—Solid unfractured pieces of coal that are bounded by cleats.

Equilibrium moisture.—Moisture content after saturating a coal sample with water at 96–97 percent relative humidity at 30°C. The humidity is maintained by placing the sample in a humidor containing a saturated solution of potassium sulfate.

Fixed carbon.—The carbon remaining after the volatile matter has been expelled during combustion.

Foreland basin.—A flexural sag in the Earth’s crust that forms from loading by thrust-sheet faulting. These basins are major sediment-accumulation sites for material being transported off of the thrust sheet and being deposited in coeval depositional systems outbound from the thrust front (i.e., shallow-sea, lake, or fluvial sedimentation).

Gas reserves.—The amount or recoverable gas determined by exploration that, under current economical and technological conditions, represents a fraction of the total gas in a reservoir.

GIP.—Acronym for gas-in-place. GIP includes all gas producible by complete desorption of coal to atmospheric pressure levels. GIP overestimates the gas resource because, realistically, a coal bed can only be depressurized to the regional pressure level, not to atmospheric pressure.

Hydrostatic gradient.—The pressure increase with depth of a liquid in contact with the surface. The hydrostatic gradient for most petroleum basins is 10.4 kPa/m (Hunt, 1979). This varies from the lithostatic gradient (pressure with depth due to overlying rock) of 24.4 kPa/m (Hunt, 1979).

Isopach map.—A contour map showing the variation of a component across a geographical area. Generally these maps show thickness of a rock unit or interval, but they can be used for other components, e.g., facies, rock type, fossil distribution, sedimentary environment.

Sorption.—A physical process where molecules of gas adhere to the surfaces of a microporous substance by weak intermolecular attraction due to van der Waals or electrostatic forces. After sorption, gas forms a liquid-like condensate in a coal, which is a microporous substance. See also Absorption or Adsorption.

Source rock.—Any rock that contains or originally contained signifi cant organic material that is currently or has undergone thermogenic or biogenic maturation. Rocks in which oil and (or) gas have been generated.

Swamps.—A wooded wetland that is ground-water and occasionally surface-water dominated.

Thermogenic gas.—Gases produced from heating a coal, usually occurring during burial but may also occur during contact metamorphism.

Thrust front.—The leading edge of a thrust sheet.

Thrust sheet.—One or more rock slabs that are thrust up a low-angle reverse fault. A thrust sheet generally implies the emplacement of older over younger rocks.

Transgressive-regressive cycle.—The raising and lowering of sea level through time with respect to a given geographic point. Various recognizable sedimentation and erosional events occur during these cycles.

Vitrinite.—The group of macerals composed of woody plant matter, mostly lignin and cellulose.

Vitrinite reflectance.—The proportion of light reflected back from a normally incident light beam of known intensity from a planar polished surface of the coal maceral vitrinite. Expressed as a percentage of the incident light returned. Abbreviated Ro.

Volatile matter.—The mass lost from a coal by heating to high temperatures in the absence of air after removal of moisture from the sample.

Water table.—A layer of water below the ground surface. It is generally used to infer the depth below the surface to which the sediments are saturated with ground water.